Heretofore, pumping systems for LC analysis equipment has been generally characterized by the use of a pump for delivering a mobile phase liquid to a separation column under elevated pressure. Injection means is interposed between the pump and the separation column for adding a liquid sample to be carried through the column by the mobile phase.
Generally, LC equipment has been designated to render the maximum possible performance over a wide variety of conditions. Typically LC equipment is designed for operating pressures above 3,000 psi and often is specified for operation from 5,000 to 10,000 psi. Equipment as specified for use at these pressures are indeed capable of nearly all LC analysis which may be required. Many pumping designs have been utilized in the past, among these include the use of syringe pumps of the order of 500 to 1,000 milliliter capacity, have been as available from Varian Aerograph of Walnut Creek, Calif.; Perkin Elmer, Inc. of Norwalk, Conn. and Isco (Instrument Specialty Company) of Nebraska.
In such liquid chromatographic applications the most common form of syringe pump has been of the size of 500 ml or higher and designed for operation at a designated pressure of 8,500 psi. Syringe pumps of this size were amongst the earlier pumps used in LC. The early success of the syringe pump relied on its mechanical simplicity and reliability and the fact that it is a positive displacement system producing very accurate flow rate deliveries. However, it came into criticism on theoretical and practical grounds because the high pressures and large volumes of these pumps were shown to be detrimental to performance. It can be shown that for a 1,000 ml volume syringe pump filled with hexane, a common hydrocarbon liquid solvent used in LC, the volume of the hexane would be compressed by an appreciable percentage greater than 1% at such pressures. Therefore, in a closed-off operation with no flow, the pump may have to move a considerable distance over an equally considerable time before normal operating pressure is achieved because the first part of the action would be taken up doing work in compressing the hexane. In dynamic situations this means that, after the pump is turned on, it could take up to twenty minutes before accurate flow rate was achieved. Further, the large size of the parts involved a certain mechanical compliance of the system which is directly proportional to scale. As an example, for 1 ml flow per minute, such pumps are operated using only one part per thousand of its total volume per minute. Thus, imperfections in the mechanical system, or its tolerances, or the compressibility errors due to operation over such a small percentage of the total capacity of the pump result in such errors being magnified. In the past it was common to manually fill such syringe pumps. This in itself was sufficiently inconvenient so as to induce manufacturers to use a large volume pump so as to reduce the refill cycle time. Thus, there is a need for a lower flow rate pump which has a better compositional accuracy of the delivered flow and which is not subject to compressibility factors and compliance factors to the degree of prior art large volume syringe pumps. In one known system an exceedingly small syringe pump having a volume of about 1 ml has been available for operation at pressures of approximately 2,000 psi.
Most other commercial LC pumps are small syringe pumps (about 0.020 to 0.10 ml) which are driven cyclically at 0.0 to 300 Hertz with a separate inlet check valve and outlet check valve, both of which are usually passively actuated. Such pump cannot produce the very low flow rates at low or moderate pressure needed for LC due to valve leakage and high flow rate design. Because these pumps are refilled and emptied many times during the course of a single LC analysis, they cause noise and spurious responses in detector output signal.
There is therefore a need for a new pumping system for liquid chromatographs which will overcome the foregoing disadvantages and limitations.